In producing integrated circuits, it is desirable to provide packaged integrated circuits having plastic or resin packages which encapsulate the die and a portion of the lead frame and its leads. These packages have been produced a variety of ways, a few of which will be described here.
Conventional molding techniques take advantage of the physical characteristics of the mold compounds. For integrated circuit package molding applications, these compounds are typically thermoset compounds. These compounds consist of an epoxy novolac resin or similar material combined with a filler, such as alumina, and other materials to make the compound suitable for molding, such as accelerators, curing agents, fillers, and mold release agents.
Transfer molding operations have three stages which correspond to the three phases of viscosity. First there is a preheat stage required to move the mold compound from its hard initial state to the low viscosity state. Second is a transfer stage, where the compound is low in viscosity and easily transported and directed into cavities and runners. This transfer process must be rapid and be completed before the mold compound begins to set. Finally there is a cure stage that occurs following the transfer stage.
There are several critical requirements that must be met in a commercially successful package molding operation. For example, the cavities must be completely and uniformly filled, the so-called "balanced fill" requirement. Using conventional single plunger molds of the prior art, the balanced fill is difficult to perform uniformly across the large mold using the single pot and the long primary runners to transport the mold compound. A problem commonly observed in a single plunger, single pot mold operation using such a mold is an unacceptable void rate. Voids are areas within the mold cavity that are not filled with compound. These can be areas where the compound fails to flow or where air or other materials are trapped and cause hollow spaces in the packaged part. Voids can be produced if the transfer rate of the mold compound is too slow during the molding process or if air or moisture is trapped in one or more the cavities during the transfer stage.
A second critical requirement is that the wire sweep defect rate be minimized below an acceptable level. Wire sweep occurs as the mold compound enters the cavity through the gates. The mold compound is dense and pulls at the fine wires that couple the bond pads of the die to the leads of the lead frame. These wires will bend under the pressure due to the flow of mold compound. As an example, suppose that in a typical lead frame and die assembly for a high pin count device, an average wire sweep after packaging of less than 6% is specified. A straight line from the lead frame lead to the bond pad has a sweep of 0%. So if after assembly and mold any wires on a packaged unit are found to have more than 6% sweep, the unit is out of specification, and is considered to be a defective unit. Wire sweep is specified as a maximum allowable parameter and is of considerable concern in production of integrated circuits, because if the bond wires are moved too much, a wire short between two or more adjacent bond wires often occurs. Alternatively, bond wires sometimes break away. Either condition results in a faulty unit.
Although the wire sweep defect rate which is observed in the single plunger molding presses is adequate for producing low to moderate pin count DIP and flat quad packaged devices, as the device pin counts continue to increase and lead frames become finer in lead to lead pitch, the wire sweep parameter becomes increasingly critical. While it is possible to build 200 pin flat quad devices using these techniques, as the pin count goes towards 400 pins and beyond, the pitch between leads will become smaller, and the prior art transfer molding presses using a single mold pot will no longer be economically suitable, due to the low yield and high wire sweep defect rates.
A further disadvantage with a single plunger mold and pellet compound arrangement is that the performance in the two critical areas are inversely dependent on each other. That is, in attempting to perfect the molding process using a single plunger mold, it has been observed that steps taken to reduce wire sweep defects typically increase the void rate, and vice versa. In other words, if the wire sweep defect rate is lowered the void rate tends to increase. The wire sweep rate can be lowered, for example, by slowing the transfer rate of the mold compound into the cavities. This tends to increase the void rate. Voids can be reduced by increasing the flow rate into the cavities, but this will tend to increase the wire sweep defect rate.
It has been further observed that the wire sweep and void problems tend to be more severe as the number of cavities and the distance of runners increases. Nonuniform fill can occur along a lengthy runner having many cavities. The cavity closest to the pot will have a faster fill rate than the others. The cavity farthest from the pot will tend to fill at the end of the transfer period, and the rate will be lower because a lot of the compound has been diverted to other cavities and because the compound is starting to harden. As a result, difficult and time consuming fine tuning of each mold press is required to establish an operation mode which will fill all of the cavities at an acceptable rate, during the low viscosity period, without increasing wire sweep defects to an unacceptable level, particularly for those cavities closest to and farthest from the mold pot.
Further, the use of the thermoset molding compound results in a process where the sprue, flash or waste that remains in the pot, the runners and between the devices themselves cannot be reused. Thermoset materials can only be used once in a molding operation, so the excess material must be discarded. Thus the sprue and waste left in the long runners and in the mold pot cannot be recycled, making waste particularly costly.
Also, the conventional molding compound acts as a strong abrasive. During molding, the mold compound is forced out of the mold pot and into the primary runners. The abrasive nature of the mold compound results in rapid wear of the mold pot and the runners, and the plunger or ram itself. This results in expensive rework or replacement of the mold chases and plungers on a frequent basis.
An alternative prior art approach for reducing the problems known to the single plunger molding presses of the prior art is to construct a multipellet, multiplunger mold station to replace the single plunger system.
In a multiplunger molding operation, each of the many mold pots receives a so called "mini-pellet" of mold compound. Each mold pot serves only a few cavities, typically one or two cavities. The press is a more complex press than that of the single plunger mold, and has a plunger for each of the mold pots. The plungers may operate from the top or from underneath the mold.
The individual plungers are used to start the transfer process, the cavities fill with mold compound as the plunger is pushed into the mold pot, and the transfer phase is completed in a few seconds.
The multiplunger mold process has some advantages over the single pot molding process. The use of the smaller pellets and the shorter runs eliminate the long runners and nonuniform fill times associated with a single plunger press. The pellets used are smaller and therefore do not require preheating, as the mold platens can provide sufficient heat to transition the mini-pellets into the low viscosity state. The wire sweep defect rate can be lowered by providing exact control of the plunger or ram insertion rate, so that the fill is done at a speed which prevents voids while minimizing wire sweep problems.
An automated multiplunger press controller can be added that can individually vary the operation of each plunger, if necessary, to obtain optimal results.
The nonuniform fill and wire sweep problems associated with the cavities nearest and farthest from the single center pot of the single plunger mold presses are reduced or eliminated. Mold compound waste is reduced by the shorter runners.
The disadvantages of the multiplunger molding process are primarily that it requires the use of the mini-pellets. The mini-pellet form of the molding compound is far more expensive than per kilogram than the single large pellets used by the single transfer mold. Also, the multiplunger molding station is extremely expensive to manufacture, operate and maintain. The automation of a press with so many plungers is more complex and expensive than the single mold press.
In addition to the added costs, the multiplunger molding station has a lower parts per hour throughput than for a conventional single pot mold press. The multiple plunger molding system requires complex control and loading and unloading mechanisms. The result is that each station has lower overall throughput than a single plunger mold station, although tighter process control can be achieved. Because the throughput is lowered, additional stations are needed to maintain the same relative level of productivity. High productivity is required to keep the unit costs low. The need for additional expensive and complex molding stations increases the cost disadvantages for the multiplunger molding systems.
Both single plunger and multiplunger mold presses have other disadvantages that are common. The mold compound is an abrasive material. The mold pot and the primary runners receive an abrasive force each time the press is operated. These areas wear quickly and the expensive mold chases must be replaced periodically as a result.
Also, both processes require pelletized mold compound. This material is fairly difficult to produce in the large form, and even more expensive to produce in the minipellet form. The compound is extruded into a rod, which is powdered, and the powder is then pelletized. This is an expensive and complex manufacturing process.
Both pellets and mini-pellets are subject to contamination by moisture and air. It is necessary to perform the molding process under pressure to elimiate trapped air and prevent the formation of voids. Moisture can become trapped in either form of pellet. Moisture contamination of the molding compound can result in additional voids and scrapped devices. Moisture contamination also contributes to package cracking during cure and afterwards to early failure of devices.
U.S. Pat. No. 5,098,626, issued Mar. 24, 1992, and entitled "Method for Packing a Measured Quantity of Thermosetting Resin And Operating a Mold for Encapsulating a Component", and herein incorporated by reference, provides another alternative wherein the mold compound is packaged in individually sealed units. These units each contain liquid mold compound in a quantity needed for a single cavity or pair of cavities for integrated circuit packages. Each package is a bag or tube containing liquid mold compound and ending in a bulge or sprout. During molding the bulge or sprout is placed at the end of a runner which feeds a cavity. As the molding process begins, the sprout is cut and the mold compound is pressed out of the bag into the cavity by individual, multiple plungers.
The '626 patent approach is similar to a conventional multiplunger mold system in that small quantities of mold compound, each of which are individually loaded, are provided. The patent provides a moisture and contamination free packaging system which can be used with an automated loading system. However, like the mini-pellets, many of these bags are required for each run. The abrasion problems are reduced, because the pots and plungers are protected by the packaging. Also, improved uniform fill and reduced wire sweep are possible. But the throughput problems and increased expense for each molding station remain, and the costs for each press are increased further by the added complexity. Also, the packaging of the mold compound in small quantities each in an individual package may lead to an expensive raw material for molding.
Accordingly, a need thus exists for a mold compound and molding system which eliminates the problems of the prior art transfer molding systems while retaining a high throughput rate, low raw material costs, and which is simple to operate, maintain, and uses molding stations that are relatively inexpensive to build. The new system should be compatible with existing single pot transfer mold presses to allow a retrofitting of existing integrated circuit assembly lines. The system should reduce waste of mold compound and reduce the abrasive impact of the mold compound on the equipment used. The new molding system should provide uniform cavity fill and reduced wire sweep defect rates.
The new molding compound should be free of impurities, air and moisture to reduce void and package cracking problems. It should be in a form that is compatible with automatic loading and unloading systems. It should be premeasured and have excellent storage durability. It should be economically competitive with the pellets and mini-pellets of the prior art. The invention described herein addresses these needs.